"We're not sure exactly of the transmission routes, where the initial infection came from, how efficient it is in transmitting," Hartl said.
None of the people who have so far been reported to have caught the swine flu had any contacts with pigs, which is how experts say the flu is passed, creating an alarm that the virus can easily spread from person to person.
Dr. Martin Blaser, a professor and chairman of the Department of Medicine at New York University Medical Center, said the situation is uncommon.
"Swine flu cases occur. They're usually in people who work with swine. This is spilled out into the general community," he said.
From across the United States, health officials are issuing strong warnings. Doctors and hospitals in California are being told by the state to report any cases with flulike symptoms. California Gov. Arnold Schwarzennegger is activating the joint emergency operations center to help deal with the threat.
Tests on 75 children who fell ill with flulike symptoms at the same time in a New York school showed that eight or nine strains have mild symptoms of "probable case of human swine influenza," New York Health Commissioner Thomas Frieden said Saturday.
Wilma J. Wooten, president of the San Diego Society of the National Medical Association, said public health experts are concerned about the outbreak.
"More than likely we will have additional cases of the swine influenza virus here in our country," she said. "We want people to stay at home if you are sick. Do not go to work. Do no go to school. Do not travel -- airplane, bus -- no form of travel."
The Centers for Disease Control's (CDC) acting director, Richard Besser, also echoed remarks made by WHO yesterday, that the outbreak could be the next flu pandemic.
"This is something we are worried about and we are treating very seriously," he said.
It has made more than a thousand people sick in Mexico, mostly between the ages of 25 and 45. In the country's capital, the mayor on Saturday cancelled all public events for the next 10 days. Mexico City was quiet as schools, libraries, museums and public buildings also closed down. People were wearing masks to prevent themselves from catching the unique virus.
The White House said President Obama is being kept up to date on the situation. The Homeland Security Council, under the leadership of terrorism czar John Brennan, is heading up the administration's handling of this situation -- and working with the CDC, the State Department and the Mexican government.